Scientists are developing new materials for artificial hips, which may prolong their life.

Existing models have an average life span of 10 to 15 years, but the "ball and socket" joint wears away with use.

Scientists in Switzerland say this happens because the artificial hip is not lubricated in the same way as a natural one.

The team, from the Swiss Federal Institute of Technology in Zurich, is now looking at whether different materials may work better.

Any research designed to improve the life-time of an implant and reduce the need of further surgery is a positive step

Spokesman for Action Research

Most UK doctors use metal "balls" and polyethylene "sockets". A combination of a ceramic ball and a polyethylene socket can also be used.

About 37,000 hip replacements are carried out in the UK every year. The operation has a failure rate of 1% per year.

Surgical materials can cost up to £1,000, with the actual operation setting the health service back another £5,000.

But the polyethylene socket is worn away over time.

In a natural joint, the synovial fluid which surrounds it lubricates the cartilage.

But in a joint made of polyethylene, the synovial fluid is less effective.

Polyethylene properties

Polyethylene is hydrophobic, meaning it repels water.

Lead scientist Professor Nicholas Spencer suspected that this property could neutralise the effectiveness of the synovial fluid as a lubricant - increasing friction and wear in the hip implant.

To test his theory, he treated the polyethylene to make it more hydrophilic, or "water-loving".

He then tested how much friction there was when a ceramic ball and a polyethylene socket were immersed in a liquid with the same properties as synovial fluid.

He found there was less friction, which would translate to less wear, than when the polyethylene was untreated.

Professor Spencer said: "Now that we've established that the nature of the polyethylene surface plays a role in how effectively it uses the proteins in synovial fluid as lubricants, we're trying to find new materials that have similar mechanical properties but are a lot less hydrophobic."

Hugh Phillips, consultant orthopaedic surgeon at Norfolk and Norwich Hospital and past president of both the British Orthopaedic Association and the British Hip Society, said the research was a welcome development

He told BBC News Online: "The problem with polyethylene is it's not lubricated.

"If you can alter the property of polyethylene to make it attract water, that rather mimics the natural lubrication of joints."

A spokesman for Action Research, a medical research charity which helped develop the UK's first artificial hip surgery, said: "Although hip replacement surgery can drastically improve the quality of life for many patients, there is some concern about its shelf life.

"Any research designed to improve the life-time of an implant and reduce the need of further surgery is a positive step."

The charity is currently funding some Leeds-based research into how artificial hips are worn away during the "swing phase" of walking, which it is hoped will highlight the benefits of different materials, and lead to recommendations to improve surgical techniques or design.

Professor Spencer's research will be presented to the American Chemical Society.